1. Field of the Invention
This invention relates to a method of and a system for controlling an idle speed in an internal combustion engine, and more particularly to a method of and a system for controlling an idle speed, in which a flowrate of air flowing through a bypass passage bypassing a throttle valve is controlled, to thereby control an engine speed to a target engine speed.
2. Description of the Prior Art
There has heretofore been the tendency of setting an idle speed at a low value in the internal combustion engines from the viewpoint of economizing the fuel consumption rate. In consequence, in the internal combustion engine being compact in size and light in weight, even if slight loads from the lighting of high beam lights and the driving of a motor fan and the like are applied during idling, the engine speed is decreased, resulting in an instable engine speed during idling. Furthermore, when a deposit adheres to the throttle valve due to the change with time, the engine speed is gradually decreased, whereby the engine speed during idling may become instable.
For this reason, there has been known a method, in which a bypass passage bypassing the throttle valve is provided, a flowrate of air flowing through this bypass passage is controlled when the throttle valve is fully closed, i.e., during the engine idling, to thereby control an engine speed to a target engine speed. An air flowrate control valve, the opening of which is controlled by a step motor, is mounted in this bypass passage, whereby the opening of the air flowrate control valve is controlled in accordance with an engine load and a shaft position, whereby a target air flowrate is fed to engine combustion chambers, the engine speed is controlled to the target engine speed or thereabout. In addition, during the operating conditions other than the full closing of the throttle valve, i.e., the operating conditions where the throttle valve is opened at a predetermined opening, the opening of the air flowrate control valve is held at a predetermined value, whereby the air-fuel mixture consisting of the air flowrate passing through the throttle valve, the air flowrate passing through the bypass passage and fuel injected from a fuel injection valve and fed to the combustion chambers of the engine is adapted to take a value in the neighborhood of the stoichiometric value.
However, in the conventional method as described above, since the opening of the air flowrate control valve is controlled by the step motor, there are presented such disadvantages that the control is complicated and the cost is raised.